Sour Gas - Hydrogen Sulfide Gas Mixture

Ariel's recommendations for sour gas service are based upon field experience and the NACE guidelines. Recommendations are to protect personnel from contact with H2S gas and materials degradation from H2S gas contact.

Recommendations:

	100 ppm to 2% H2S Sour Level 1	> 2% H2S Sour Level 2
Distance Pieces / Crosshead Guides	S2C / L2C	L2C
Piston Rod Material	Stainless Steel	Stainless Steel
Purge Packing	Purged	Purged
Flushing Lube	Flushing Lube	Flushing Lube
Stainless Steel Valves	Standard Valves	Stainless Steel
Forged Steel (-VS) Cylinders	Use -VS-H2S or Stainless Bodies Even as Low as 5-10 ppm of H2S Content

Optional Equipment:

Cylinder Bolting Material
Cylinder Body Material
Lubrication Tubing and Fitting Materials
Separate Lube Supply

Recommendations for Sour Gas Service:

Distance Piece / Crosshead Guide: Two compartment guides are recommended for sour gas service. The two compartments offer the availability to create a purge and vent system to segregate the frame compartment and operator atmosphere from the packing leakage / blowby. Short two compartment distance piece/crosshead guides, "S2C", can be used for 100 ppm H2S content up to 2% H2S content. Long two compartment distance pieces, "L2C", are recommended above 2% H2S content as this provides better containment of the gas leakage. S2C guides can be used above 2% H2S content, but do not offer the same level of protection and flexibility for the vent / purge system design. Two compartment guides are available for all units except for the JGM:N:P:Q frames. Some cylinder and frame classes do not have the availability of a S2C guide, please refer to the performance program for S2C availability on each frame or cylinder (JGU:Z, KBB:V cylinders and some -VS cylinders). When the S2C guide is not offered by Ariel, the long single guide may be suitable for Sour Level 1 applications with purged packing cases. Refer to the Distance Piece Purge and Vent Arrangements for notes on a purge system for sour gas.

Piston Rod Material: Ariel standard piston rod material is 4100 series carbon steel heat treated for strength. These are not suitable for sour gas service as defined by NACE. For sour gas service with 100 ppm and greater H2S content stainless steel piston rods are recommended. Ariel stainless steel piston rod material is 17-4PH in the double H1150 condition, UNS S17400, (per NACE procedure #1), Rc 33 maximum, chromium-nitrided in the packing travel area for a surface hardness of 2500 Vickers. The chromium nitride process has a length limitation due to manufacturing constraints. Longer stainless steel piston rods will be tungsten carbide coated.

Purge Packing: Ariel recommends the use of "Purge packing" for sour gas services with 100 ppm and greater H2S content. Purge packing refers to a packing case modified to accept an external purge pressure 15 to 20 psi above the primary packing vent/drain pressure with no more than 5 psi external system back pressure. The purge gas must be sweet natural gas, propane, or an inert gas, such as nitrogen. The purpose of purge gas is to block and contain hazardous, toxic, flammable or corrosive gases, and to prevent such from entering the compressor frame where damage to the running gear, or personnel safety hazards can occur. Refer to the Distance Piece Purge and Vent Arrangements for notes on a purge system for sour gas. See also "Packing Leakage".

Flushing Lube: Ariel recommends the use of cylinder flushing lube for sour gas services with 100 ppm and greater H2S content. Flushing oil is available as an option on all units. Flushing oil is used to ensure corrosion protection for the suction valves, both the valve bodies and springs. Ariel would supply additional distribution blocks so that oil can be injected into the suction nozzle riser of the cylinder. Flushing lube lines will be tubed to the end of the crosshead guide. The Packager would provide the lube lines from the end of the guide to the suction pulsation vessels cylinder nozzle. An atomizer for flushing lube injection can be constructed of tubing inserted into the gas stream at the suction pulsation cylinder nozzle with the end opened with a diagonal cut.

Stainless Steel Valves: Ariel recommends the use of stainless steel valves for sour gas services with 2% and greater H2S content. Stainless steel valves generally consist of:
seats and guards: 17-4PH - H1150
plates: Nylon, MT or PEEK. 17-7PH or 15-7PH if metallic plates
coil springs: Nimonic 90
spring plates: stainless steel (equivalent to 420)

It could be prudent to start up a unit with standard steel valves and replace those with the stainless steel valves after any start up problems have been solved and piping debris has passed through the unit.

Optional Equipment for Sour Gas Service:

Cylinder bolting: Standard cylinder bolting for sweet and sour gas services consists of Grade 8 valve cap, head end head, crank end head and packing case cap screws. Cylinder bolts are outside the gas containment and not considered "wetted". Ariel provides an option to replace these bolts with 17-4PH stainless steel bolts for the valve caps, head end head, crank end head and packing case bolting.

Cylinder Body Materials: Standard cylinder body materials are:
ASTM A278 Grade 40 gray iron
ASTM A536 Ductile iron 80-55-06
ASTM A395 Ductile iron 60-40-18
AISI 4340 Forged steel
ASTM A668 Class J forged steel

Forged Steel Cylinder Body Materials: Attention must be paid to forged steel cylinder selections for hydrogen sulfide content. The NACE MR-0175 Stress Cracking Region will be used to determine how much H2S is allowed for AISI 4340 cylinder bodies. Most Ariel forged steel cylinder bodies are manufactured from AISI 4340 high strength carbon steel. This material is not suitable for H2S content, even as low as 5 to 10 ppm. When even small amounts of H2S is present, forged steel cylinders will need to be either softened to 22 RC or manufactured of 17-4PH Stainless steel. This will impact most Ariel double acting forged steel cylinders.

Double acting forged steel cylinders that have an -H2S designation in the cylinder class are suitable for sour gas content. All forged steel tandem cylinder arrangements are suitable for H2S content due to the controlled hardness of 22 RC maximum.

Lube Tubing Ariel standard cylinder and frame oil system is 304 stainless steel tubing with zinc plated carbon steel fittings. An option is available to use 316 stainless steel tubing and 316 stainless steel fittings. This would encompass the frame and cylinder tubing and fittings on a unit. A separate option is available for frame oil piping on the suction side of the main oil pump on the larger units where welded piping is used in lieu of tubing. Fittings and plugs outside the frame and cylinder oil flow are not included in these stainless steel options. (See Price Book for possible exceptions.)

Separate Lube Supply: Provision is made on all units to use a separate oil source for the cylinder bores and packing cases. Ariel standard is to tube from the main frame oil header to the inlet of the cylinder lubricator pumps. The packager can remove this tube, plug the connection at the main frame oil header and connect the separate supply to the inlet of the lubricator pumps. Sour gas services typically require a special lubricant through this separate lube supply. Refer to Section 6 of the Packager's Standards for lubrication recommendations.

Ariel Corporation Application Manual
28 December 2007